Method of transmitting control signals in wireless communication system

ABSTRACT

A method of transmitting control signals in a wireless communication system includes multiplexing a first control signal with a second control signal in a slot, the slot comprising a plurality of orthogonal frequency division multiplexing (OFDM) symbols in time domain, the plurality of OFDM symbols being divided into a plurality of data OFDM symbols and a plurality of reference signal (RS) OFDM symbols, wherein the first control signal is mapped to the plurality of data OFDM symbols after the first control signal is spread by a base sequence in the frequency domain, the RS is mapped to the plurality of RS OFDM symbols, the second control signal is mapped to at least one of the plurality of RS OFDM symbols, and transmitting the first control signal and the second control signal in the slot.

This application is a continuation of U.S. application Ser. No.12/451,049, filed Oct. 23, 2009, which is a U.S. National Phase entry ofInternational Application No. PCT/KR2008/003339, filed on Jun. 13, 2008,and claims the benefit of U.S. Provisional Application No. 60/944,074,filed Jun. 14, 2007 and Korean Patent Application No. 10-2008-0009192,filed on Jan. 29, 2008, each of which are hereby incorporated byreference as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method of transmitting control signals in a wirelesscommunication system.

BACKGROUND ART

In order to maximize efficiency under a limited radio resource in awideband wireless communication system, methods for more effectivelytransmitting data in time, spatial, and frequency domains have beenprovided.

Orthogonal frequency division multiplexing (OFDM) uses a plurality oforthogonal subcarriers. Further, the OFDM uses an orthogonality betweeninverse fast Fourier transform (IFFT) and fast Fourier transform (FFT).A transmitter transmits data by performing IFFT. A receiver restoresoriginal data by performing FFT on a received signal. The transmitteruses IFFT to combine the plurality of subcarriers, and the receiver usesFFT to split the plurality of subcarriers. According to the OFDM,complexity of the receiver can be reduced in a frequency selectivefading environment of a broadband channel, and spectral efficiency canbe increased when selective scheduling is performed in a frequencydomain by using a channel characteristic which is different from onesubcarrier to another. Orthogonal frequency division multiple access(OFDMA) is an OFDM-based multiple access scheme. According to the OFDMA,efficiency of radio resources can be increased by allocating differentsubcarriers to multiple users.

To maximize efficiency in the spatial domain, the OFDM/OFDMA-basedsystem uses a multiple-antenna technique which is used as a suitabletechnique for high-speed multimedia data transmission by generating aplurality of time/frequency domains in the spatial domain. TheOFDM/OFDMA-based system also uses a channel coding scheme for effectiveuse of resources in the time domain, a scheduling scheme which uses achannel selective characteristic of a plurality of users, a hybridautomatic repeat request (HARQ) scheme suitable for packet datatransmission, etc.

In order to implement various transmission or reception methods toachieve high-speed packet transmission, transmission of a control signalon the time, spatial, and frequency domains is an essential andindispensable factor. A channel for transmitting the control signal isreferred to as a control channel. An uplink control signal may bevarious such as an acknowledgement (ACK)/negative-acknowledgement (NACK)signal as a response for downlink data transmission, a channel qualityindicator (CQI) indicating downlink channel quality, a precoding matrixindex (PMI), a rank indicator (RI), etc.

In general, a control channel uses more limited time/frequency resourcesthan those used in a data channel. State information of a radio channelneeds to be fed back in order to increase spectral efficiency of asystem and a multi-user diversity gain. Therefore, effective design ofthe control channel is inevitable when large-sized data is fed back. Inaddition, the control channel has to be designed to have a goodpeak-to-average power ratio (PAPR)/cubic metric (CM) characteristic inorder to reduce power consumed in a user equipment.

There is a need for a control channel structure capable of keeping goodPAPR/CM characteristics while increasing transmission capacity.

DISCLOSURE OF INVENTION Technical Problem

The present invention provides a method of simultaneously transmittingdifferent control signals by using allocated time/frequency resources.

The present invention also provides a method of transmitting a pluralityof control signals through a control channel.

Technical Solution

In an aspect, a method of transmitting control signals in a wirelesscommunication system includes multiplexing a first control signal with asecond control signal in a slot, the slot comprising a plurality oforthogonal frequency division multiplexing (OFDM) symbols in timedomain, the plurality of OFDM symbols being divided into a plurality ofdata OFDM symbols and a plurality of reference signal (RS) OFDM symbols,the plurality of data OFDM symbols used to transmit the first controlsignal, the plurality of RS OFDM symbols used to transmit a RS, whereinthe first control signal is mapped to the plurality of data OFDM symbolsafter the first control signal is spread by a base sequence in thefrequency domain, the RS is mapped to the plurality of RS OFDM symbols,the second control signal is mapped to at least one of the plurality ofRS OFDM symbols, and transmitting the first control signal and thesecond control signal in the slot.

A subframe can include two slots and each of the two slots in thesubframe can use different subcarriers.

The first control signal can be a channel quality indicator (CQI) whichrepresents downlink channel condition and the second control signal canbe an acknowledgement (ACK)/negative-acknowledgement (NACK) signal forhybrid automatic repeat request (HARM).

The slot can include five data OFDM symbols and two RS OFDM symbols, andthe two RS OFDM symbols are not contiguous with each other. The secondcontrol signal can be mapped to the last RS OFDM symbol in the slot.

The first control signal can use quadrature phase shift keying (QPSK)modulation and the second control signal can use QPSK or binary phaseshift keying (BPSK) modulation.

In another aspect, a method of transmitting control signals in awireless communication system includes transmitting a first controlsignal and a second control signal on an uplink control channel, a slotbeing used to transmit the uplink control channel, the slot comprising aplurality of OFDM symbols, wherein the second control signal ismultiplexed with a RS which is used for the first control signal, thefirst control signal and the RS are transmitted in different OFDMsymbols on the uplink control channel, and the second control signal istransmitted with the RS in one of OFDM symbols used for the RS, andwherein the first control signal uses QPSK modulation and the secondcontrol signal uses QPSK or BPSK modulation.

In still another aspect, a method of transmitting control signals in awireless communication system includes configuring an uplink controlchannel carrying a first control signal and a second control signal,wherein the first control signal and a RS are transmitted in differentOFDM symbols on the uplink control channel and the second control signalis multiplexed with the RS, and transmitting the first control signaland the second control signal on the uplink control channel.

Advantageous Effects

Transmission capacity for an uplink control channel can be increased,and a peak-to-average power ratio (PAPR)/cubic metric (CM)characteristic can be preserved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 is a block diagram showing a transmitter according to anembodiment of the present invention.

FIG. 3 shows an exemplary structure of a radio frame.

FIG. 4 shows an exemplary subframe.

FIG. 5 shows a structure of a channel quality indicator (CQI) channel.

FIG. 6 shows a structure of a control channel according to an embodimentof the present invention.

FIG. 7 shows a structure of a control channel according to anotherembodiment of the present invention.

FIG. 8 shows a structure of a control channel according to anotherembodiment of the present invention.

FIG. 9 shows a structure of a control channel according to anotherembodiment of the present invention.

FIG. 10 shows an example of control signal transmission when multipleresource blocks are assigned.

FIG. 11 shows another example of control signal transmission whenmultiple resource blocks are assigned.

FIG. 12 shows an example of control signal transmission when multipleresource blocks are assigned.

FIG. 13 is a flowchart showing a method of generating a reserved signalmapped to a reserved subcarrier.

FIG. 14 shows an example of control signal transmission using a longspreading code.

FIG. 15 shows a structure of an ACK/NACK channel.

MODE FOR THE INVENTION

FIG. 1 shows a wireless communication system. The wireless communicationsystem can be widely deployed to provide a variety of communicationservices, such as voices, packet data, etc.

Referring to FIG. 1, a wireless communication system includes at leastone user equipment (UE) 10 and a base station (BS) 20. The UE 10 may befixed or mobile, and may be referred to as another terminology, such asa mobile station (MS), a user terminal (UT), a subscriber station (SS),a wireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the UE 10 and may be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), anaccess point, etc. There are one or more cells within the coverage ofthe BS 20.

Hereinafter, a downlink is defined as a communication link from the BS20 to the UE 10, and an uplink is defined as a communication link fromthe UE 10 to the BS 20. In the downlink, a transmitter may be a part ofthe BS 20, and a receiver may be a part of the UE 10. In the uplink, thetransmitter may be a part of the UE 10, and the receiver may be a partof the BS 20.

FIG. 2 is a block diagram showing a transmitter according to anembodiment of the present invention.

Referring to FIG. 2, a transmitter 100 includes a transmit (Tx)processor 110, a discrete Fourier transform (DFT) unit 120 that performsDFT, and an inverse fast Fourier transform (IFFT) unit 130 that performsIFFT. The DFT unit 120 performs DFT on data processed by the Txprocessor 110 and outputs a frequency-domain symbol. The data input tothe DFT unit 120 may be a control signal and/or user data. The IFFT unit130 performs IFFT on the received frequency-domain symbol and outputs aTx signal. The Tx signal is a time-domain signal and is transmittedthrough a Tx antenna 190. A time-domain symbol output from the IFFT unit130 is referred to as an orthogonal frequency division multiplexing(OFDM) symbol. Since IFFT is performed after DFT spreading, thetime-domain symbol output from the IFFT unit 130 is also referred to asa single carrier-frequency division multiple access (SC-FDMA) symbol.SC-FDMA is a scheme in which spreading is achieved by performing DFT ata previous stage of the IFFT unit 130 and is advantageous over the OFDMin terms of decreasing a peak-to-average power ratio (PAPR)/cubic metric(CM).

FIG. 3 shows an exemplary structure of a radio frame.

Referring to FIG. 3, a radio frame includes 10 subframes. One subframecan include two slots. One slot can include a plurality of OFDM symbolsin a time domain and at least one subcarrier in a frequency domain. Theslot is a unit of radio resource allocation in the time domain. Forexample, one slot can include 7 or 6 OFDM symbols. A resource block isdefined by a slot in the time domain and a plurality of subcarriers inthe frequency domain, and is a basic unit of radio resource allocation.It is assumed hereinafter that one resource block is defined by one slotand 12 subcarriers.

The radio frame structure is shown for exemplary purposes only, and thusthe number of subframes included in the radio frame or the number ofslots included in the subframe or the number of OFDM symbols included inthe slot is not limited thereto.

FIG. 4 shows an exemplary subframe. The subframe may be an uplinksubframe using SC-FDMA. A time for transmitting one subframe is definedas a transmission time interval (TTI).

Referring to FIG. 4, an uplink subframe can be divided into two parts,that is, a control region and a data region. Since the control regionand the data region use different frequency bands, frequency divisionmultiplexing (FDM) have been achieved.

The control region is used to transmit only the control signal and isassigned to a control channel. The data region is used to transmit dataand is assigned to a data channel. The control channel transmits thecontrol signal. The data channel transmits the user data and/or thecontrol signal. The control channel and the data channel can beconfigured within one subframe. However, in order to keep asingle-carrier property, the control channel and the data channel cannotbe simultaneously transmitted by one UE within one subframe. The controlchannel can be referred to as a physical uplink control channel (PUCCH).The data channel can be referred to as a physical uplink shared channel(PUSCH). Examples of the control signal include an acknowledgement(ACK)/negative-acknowledgement (NACK) signal for hybrid automatic repeatrequest (HARQ), a channel quality indicator (CQI) indicating a downlinkchannel condition, a precoding matrix index (PMI) indicating a precodingmatrix of a codebook, a rank indicator (RI) indicating the number ofindependent multiple input multiple output (MIMO) channels, a schedulingrequest (SR) for requesting uplink radio resource allocation, etc.

The control signal is carried on the control region. The user data andthe control signal can be carried together on the data region. That is,when the UE transmits only the control signal, the control region can beassigned to transmit the control signal. In addition, when the UEtransmits both the data and the control signal, the data region can beassigned to transmit the data and the control signal. In an exceptionalcase, even if only the control signal is transmitted, the control signalmay be transmitted in a large amount or the control signal may be notsuitable to be transmitted through the control region. In this case, aradio resource can be assigned to the data region to transmit thecontrol signal.

In the control region, control channels of respective UEs can usedifferent frequencies (or subcarriers) or different codes. Frequencydivision multiplexing (FDM) or code division multiplexing (CDM) may beused to identify each control channel.

Each of two slots included in one subframe can be frequency hopped. Thatis, one of the two slots included in one subframe is assigned to oneside of a frequency band, and the other slot is assigned to the otherside of the frequency band. A frequency diversity gain can be obtainedby transmitting each control channel through the slots, each of whichuses a different subcarrier.

For clarity, it will be assumed hereinafter that one slot consists of 7OFDM symbols, and thus one subframe including two slots consists of 14OFDM symbols in total. The number of OFDM symbols included in onesubframe or the number of OFDM symbols included in one slot is shown forexemplary purposes only, and thus the technical scope of the presentinvention is not limited thereto.

FIG. 5 shows a structure of a CQI channel. The CQI channel is used totransmit a CQI.

Referring to FIG. 5, one slot includes 7 OFDM symbols. A referencesignal (RS) is assigned to two of the 7 OFDM symbols, and the CQI isassigned to the remaining 5 OFDM symbols. An OFDM symbol mapped to theCQI is referred to as a ‘data OFDM symbol’. An OFDM symbol mapped to theRS is referred to as an ‘RS OFDM symbol’. The location and the number ofRS OFDM symbols may vary depending on a control channel. Changes in thelocation and the number of RS OFDM symbols may result in changes inthose of data OFDM symbols.

When the control signal is transmitted on the CQI channel,frequency-domain spreading is used to increase the number ofmultiplexable UEs or the number of control channels. A frequency-domainspreading code is used to spread the CQI. A Zadoff-Chu (ZC) sequence isone example of a constant amplitude zero autocorrelation (CAZAC)sequence and is used as the frequency-domain spreading code. If the CQIchannel is assigned with one resource block, a CAZAC sequence having alength of 12 is used.

A ZC sequence c(k) having a length of N can be generated as shown:

${c(k)} = \left\{ \begin{matrix}^{{- j}\frac{\pi \; {{Mk}{({k + 1})}}}{N}} & {{for}\mspace{14mu} {odd}\mspace{14mu} N} \\^{{- j}\frac{\pi \; {Mk}^{2}}{N}} & {{for}\mspace{14mu} {even}\mspace{14mu} N}\end{matrix} \right.$

where 0≦k≦N−1, and M is a root index and is a natural number equal to orless than N. N is relatively prime to M. This means that, once N isdetermined, the number of root indices is equal to the number ofavailable ZC sequences. UEs can be identified by using ZC sequences eachhaving a different cyclic shift value. The number of possible cyclicshifts may vary depending on channel delay spread.

If the CQI channel is assigned with one resource block including 12subcarriers and if 6 cyclic shifts are possible for the ZC sequence,then 6 UEs can be identified. If a CQI using quadrature phase shiftkeying (QPSK) modulation is mapped to each OFDM symbol, a 10-bit codedCQI can be transmitted in every slot. That is, a maximum of 10-bit CQIcan be transmitted through one subframe. For example, 5 subframes arerequired to transmit a 50-bit CQI. When two or more resource blocks areassigned, the ZC sequence is increased in length and thus an additionalspreading gain can be obtained. However, there is no change in thenumber of supportable UEs and transmission capacity. Accordingly, thereis a need for a method in which different control signals aresimultaneously transmitted by dividing allocated frequency resources toimprove transmission capacity while maintaining the PAPR/CMcharacteristic.

Radio resources of a spatial domain in addition to time/frequencydomains can be effectively utilized by transmitting various uplinkcontrol signals. Example of the various control signals to betransmitted include not only a large-sized control signal (i.e., CQI)but also relatively small-sized other control signals (i.e., ACK/NACK,SR, PMI, RI, etc.). The control signals can be transmitted throughindependent channel allocation. However, due to a characteristic of aspreading code, the PAPR/CM characteristic may be problematic when aplurality of control channels are simultaneously transmitted. Inparticular, since a MIMO-related control signal has a correlation withthe CQI, the control signal may be preferably mapped to the CQI channelwhen transmitted. A 1 or 2 bit-control signal (i.e., ACK/NACK or SR) maybe mapped to the large-sized control channel in order to increasespectral efficiency.

Structure of Multiplexed Control Channel

Now, a method of multiplexing a small-sized control signal (e.g.,ACK/NACK, SR, etc.) through a control channel (e.g., CQI channel) fortransmitting a large-sized control signal will be described. Thesmall-sized control signal denotes a control signal whose size issmaller than the large-sized control signal. For example, thesmall-sized control signal may be a control signal having a small numberof bits. However, the size of the control signal is not particularlyrestricted in the present invention.

Required transmission capacity differs according to a type and purposeof an uplink control signal. For example, feedback of channelinformation for a narrow band is required to obtain frequency andmulti-user diversity gains through frequency selective scheduling.Therefore, when a CQI is transmitted in a wideband system, informationhaving a size in the range of several bits to tens of bits istransmitted for a unit time (e.g., 1 TTI). On the contrary, when anACK/NACK signal, an SR, a MIMO-related PMI, an RI, etc., aretransmitted, information having a size in the range of 1 or 2 bits toseveral bits is transmitted if necessary. In case of the small-sizedcontrol signal, supportable UE capability may be more important than atransmittable symbol interval of a unit time duration. In addition, whendifferent control signals are transmitted through a plurality of controlchannels, the PAPR/CM characteristic may deteriorate due to a spreadingcode property that maintains an excellent PAPR/CM within a unit channel.

FIG. 6 shows a structure of a control channel according to an embodimentof the present invention. With this structure, a small-sized controlsignal is multiplexed and transmitted through a CQI channel that can beregarded as a large-seized control channel.

Referring to FIG. 6, the CQI channel uses a spreading code based on a ZCsequence in a frequency domain. UEs are multiplexed by utilizing amaximum of 6 orthogonal codes by performing a cyclic shift. Accordingly,a CQI can be transmitted using 5 OFDM symbols in every slot.

The CQI channel uses two RSs for coherence detection. Small-sizedcontrol signals are mapped to two RS OFDM symbols. That is, the RSs aremultiplexed with the small-sized control signals. A maximum of 12orthogonal codes can be obtained by using two RS OFDM symbols. That is,a maximum of 12 two-dimensional orthogonal codes can be obtained byusing an orthogonal spreading code (e.g., Walsh-Hadamard (W-H) code) ina time domain and by using 6 ZC sequences that can be obtained byperforming 6 cyclic shifts in the frequency domain.

A plurality of bits per unit of transmission can be transmitted byselecting a different code in every slot. In addition, a diversity gaincan be obtained through frequency hopping by selecting the same code inevery slot. For example, if it is assumed that a 1-bit ACK/NACK signalor a 1-bit SR is used, an orthogonal code of (1,1) or (1,−1) may betransmitted by selecting a bit 0 (i.e., ACK) or a bit 1 (i.e., NACK) ormay be carried and transmitted on a RS by selecting an ACK signal (i.e.,(1,−1)) or a NACK signal (i.e., (−1,−1)). Further, if a 2-bit ACK/NACKsignal is used, (1,1), (1,−1), (−1,−1), and (−1,1) may be used for(NACK,NACK) or discontinuous transmission (DTX), (ACK,ACK), (ACK,NACK),and (NACK,ACK), respectively.

The spreading code can be processed at a previous stage of processingthe ZC sequence as shown in the figure. However, since a characteristicof the ZC sequence is maintained even after IFFT is performed,transmission may be performed through multiplication of the spreadingcode after the IFFT is performed.

Multiplexing of the small-sized control signal does not have an effecton transmission capacity of the CQI channel and UE capability. Thesmall-sized control signal can use the spreading code in order to bemultiplexed with a RS of the CQI channel (hereinafter simply referred to‘CQI RS’). In addition, the small-sized control signal can bemultiplexed with the RS according to a modulation scheme using symbolsof the small-sized control signal. For example, the RS may bemultiplexed with a binary phase shift keying (BPSK) or QPSK-modulatedACK/NACK signal.

Each slot may use a different spreading code. Alternatively, two slotsmay use the same spreading code. A long spreading code can be usedthroughout a plurality of slots. For example, a spreading code having alength of 4 may be used for 4 RSs in two slots.

If the small-sized control signal is transmitted together with a CQI RS,coherent detection is used for the CQI and non-coherent detection isused for multiplexed other control signals. This is because thesmall-sized control signal is mapped to a RS OFDM symbol. A receiver mayfirst reproduce a multiplexed control signal mapped to the RS OFDMsymbol through non-coherent detection and then reproduce the CQI throughcoherent detection. If a two-dimensional orthogonal code is used, thereceiver can reproduce the multiplexed control signal by performing ade-spreading process. If the control signal is multiplexed with the RS,coherent detection of the CQI may be influenced in terms of performance.However, loss can be minimized if the multiplexed control signal is asmall-sized control signal. For example, if the multiplexed controlsignal has a size of 1 bit and if an orthogonal code of (1,1) or (1,−1)is transmitted by selecting a bit 0 or a bit 1, then the same data,i.e., 1, is transmitted for a first RS OFDM symbol. Therefore, coherentdetection performance can be maintained at least with respect to one RS.Deterioration of coherent detection performance can be minimized bydetermining some of a plurality of RSs to “dedicated RSs” and theremaining RSs to “relative RS s”.

Although two-dimensional orthogonal codes of the time domain and thefrequency domain are used to multiplex the control signal,one-dimensional codes may also be used such as time-domain orthogonalcodes or frequency-domain orthogonal codes.

FIG. 7 shows a structure of a control channel according to anotherembodiment of the present invention. With this structure, a small-sizedcontrol signal is multiplexed and transmitted through a CQI channel thatcan be regarded as a high-sized control channel.

Referring to FIG. 7, a CQI and a RS use different OFDM symbols. Thesmall-sized control signal is multiplexed with one of a plurality ofOFDM symbols to be assigned to the RS. Two RS OFDM symbols are assignedto the CQI channel. Between the two RS OFDM symbols, a first RS OFDMsymbol is assigned to a CQI RS, and a second RS OFDM symbol is mappedwith a small-sized control signal (e.g., an ACK/NACK signal) togetherwith the CQI RS. That is, the small-sized control signal is multiplexedwith the RS in the second RS OFDM symbol. For example, the CQI RS may bemodulated and multiplexed by using a BPSK (i.e., 1-bit ACK/NACK) or QPSK(i.e., 2-bit ACK/NACK) symbol mapped to the ACK/NACK signal.Specifically, in case of a CQI channel that transmits 10-bit informationbits by performing channel coding at a half code rate, 10 QPSK CQImodulation signals d(0) to d(9) are mapped to 5 OFDM symbols andtransmitted in every slot. In this case, if an ACK/NACK signal of one ortwo bits is modulated using BPSK or QPSK, a single modulation symbol(i.e., d(10)) can be multiplexed with a RS and thus a maximum of 21 bitsor 22 bits can be transmitted through one subframe (i.e., twoconsecutive frequency hopping slots). For example, the ACK/NACK signalcan be multiplexed with the RS in such a manner that one modulatedsymbol is multiplied with a RS sequence.

Each slot may use a different modulation symbol. The same modulationsymbol may be used in two slots. Although a RS is modulated at aprevious stage of IFFT processing, it may be modulated in a next stageof the IFFT processing.

The location of the RS OFDM symbol to which the small-sized controlsignal is multiplexed is shown for exemplary purposes only, and thus thepresent invention is not limited thereto. Not only the second RS OFDMsymbol but also the first RS OFDM symbol can be multiplexed, and thelocation of the OFDM symbol multiplexed in every slot may change.

FIG. 8 shows a structure of a control channel according to anotherembodiment of the present invention. With this structure, a small-sizedcontrol signal is multiplexed and transmitted through a CQI channel thatcan be regarded as a large-sized control channel.

Referring to FIG. 8, the small-sized control signal is multiplexedthrough phase-shift in consideration of a modulation scheme of a CQIthat is a large-sized control signal. In this case, additional powerallocation is not necessary and bandwidth loss does not occur. Forexample, if there is no phase shift, only the CQI may be transmitted,whereas if there is a phase shift of π/4, an SR may be transmitted bybeing multiplexed with the CQI.

The phase shift can be set differently for each slot, thereby increasingtransmission capacity. The same phase-shift may be performed for 1 TTIor more so as to obtain a frequency diversity gain through frequencyhopping and a time diversity gain through repetition.

FIG. 9 shows a structure of a control channel according to anotherembodiment of the present invention. With this structure, a small-sizedcontrol signal is multiplexed and transmitted through a CQI channel thatcan be regarded as a large-sized control channel.

Referring to FIG. 9, a specific sequence is selected according to thesmall-sized control signal, and the selected sequence is used as amasking code. It is assumed for example that a ZC sequence has a lengthof 12, corresponding to a length of one resource block. In addition, aW-H code having a length of 4 is used for a 2-bit small-sized controlsignal. One of the 4 W-H codes is selected as the masking code for thesmall-sized control signal. The ZC sequence having a length of 12 issegmented into 3 parts, and each segment is masked using the maskingcode. The masked sequence is used as a frequency-domain spreading codeand thus is transmitted by expanding a CQI. The small-sized controlsignal is modulated through sequence modulation while maintaining aconventional characteristic of the ZC sequence. A receiver reproduces amultiplexed control signal by performing a de-spreading process on asequence-modulated code, and then produces a CQI.

A diversity gain can be obtained when the same small-sized controlsignal is transmitted in one slot duration. Alternatively, a differentsmall-sized control signal may be used for each OFDM symbol.

When the masking code is masked onto the ZC sequence, orthogonality maybe influenced by a cyclic shift of the ZC sequence. As a result, acyclic shift offset may be restricted. However, it is possible tocontrol supportable UE capability and transmission capacity for a unittime period. For example, if a different control signal is mapped foreach OFDM symbol through the W-H code having a length of 4, a maximum of20 bits can be transmitted for one slot. This means that transmissioncapacity is approximately doubled.

Although the W-H code and the ZC sequence are described for sequencemodulation as an example, other general orthogonal codes may also beused.

Transmission of Control Signal Using Multiple Resource Blocks

When a plurality of resource blocks (RBs) are assigned to transmit anuplink control signal, a frequency-domain spreading code is adjusted tofit a size of the assigned RBs in order to maintain a single carrierproperty. In the control channel structure shown in FIG. 5, the numberof supportable UEs and transmission capacity per unit time are limitedregardless of the number of assigned RBs. For example, a control channelconstructed of two RBs supports UE capability of 6 and transmissioncapacity of 5 OFDM symbols per slot, which is the same as thosesupported by a control channel constructed of one RB. Therefore,according to the conventional control channel structure, transmissioncapacity cannot be increased even if a frequency resource isadditionally allocated. As a result, spectral efficiency decreases.Accordingly, when a plurality of RBs are assigned, there is a need for acontrol channel structure capable of increasing transmission capacity byallocating additional frequency resources while maintaining an excellentPAPR/CM characteristic.

FIG. 10 shows an example of control signal transmission when multipleRBs are assigned.

Referring to FIG. 10, when k RBs are assigned, k spreading codes areassigned. A control signal is mapped for each spreading code at aprevious stage of DFT processing. A DFT-spread code is mapped to asubcarrier. Thereafter, the resultant code is subjected to IFFT and isthen transmitted. Consequently, k control signals can be transmitted byassigning the k RBs. The spreading code can be used for intra-cell UEidentification and/or cell identification. The same spreading code maybe used. Alternatively, a different code may be used according to acontrol signal mapped thereto.

Although one spreading code is assigned for one RB herein, one spreadingcode may be assigned for a plurality of RBs. For example, one spreadingcode may be assigned for two RBs.

Transmission capacity increases k times higher than the conventionalchannel. According to required transmission capacity, spectralefficiency can be maximized through flexible frequency allocation.

FIG. 11 shows another example of control signal transmission whenmultiple RBs are assigned.

Referring to FIG. 11, in comparison with the embodiment of FIG. 10,control signals are spread through a spreading code at a previous stageof DFT processing and then are interleaved by using an interleaver. Theinterleaved signal is spread through DFT and is then mapped to asubcarrier. Thereafter, the resultant signal is subjected to IFFT and isthen transmitted.

FIG. 12 shows an example of control signal transmission when multipleRBs are assigned.

Referring to FIG. 12, if one RB includes 12 subcarriers, each of aplurality of RBs uses a ZC sequence having a length of 11 as a spreadingcode. A random code is assigned to the remaining one reserved subcarrierin order to improve the PAPR/CM characteristic. For K RBs, onesubcarrier is reserved for each RB. Thus, random codes are assigned to Kreserved subcarriers in order to improve the PAPR/CM characteristic.

The ZC sequence having a length of 11 may be directly used withoutalternation. A ZC sequence having a length of 12 may be truncated to beused as the ZC sequence having a length of 11. A ZC sequence having alength of less than 11 may be extended to be used as the ZC sequencehaving a length of 11.

Although a first subcarrier among 12 subcarriers constituting each RB isused as the reserved subcarrier, there is no restriction on the locationof the reserved subcarrier. Thus, the reserved subcarrier may bepositioned at a last subcarrier or a middle portion of each RB. Inaddition, one or more reserved subcarriers can be assigned for each RB.The number of reserved subcarriers assigned for the RBs may differ fromone RB to another.

When one RB (i.e., 12 subcarriers) is assigned to the control channel inthe conventional control channel structure, a ZC sequence having alength of 12 is used as a spreading code. If two RBs are assigned, a ZCsequence having a length of 24 is used as the spreading code. When a ZCsequence corresponding to one RB is used for two RBs, the PAPR/CMcharacteristic of the ZC sequence may be impaired. Accordingly, some ofthe assigned subcarriers are used for the purpose of improving thePAPR/CM characteristic.

A computational amount can be too high when the random codes arecalculated for every transmission unit. In this case, a look-up tablecan be used. In general, a control signal uses a predeterminedmodulation scheme (e.g., QPSK) and a ZC sequence has a predeterminedlength. Therefore, a random signal to be mapped to a reserved subcarrieris pre-stored in the look-up table so that an excellent PAPR/CMcharacteristic can be provided according to transmittable data (i.e.,the control signal).

FIG. 13 is a flowchart showing a method of generating a reserved signalmapped to a reserved subcarrier.

Referring to FIG. 13, in step S310, reserved subcarriers are selectedfrom assigned resource blocks. In step S320, data is generated using apredetermined modulation scheme. In step S330, a sequence is selected.For example, when a resource block includes 12 subcarriers and uses onesubcarrier as a reserved subcarrier, a ZC sequence having a length of 11is selected. In step S340, a Tx signal is generated using data or theselected sequence. In step S350, the generated Tx signal is comparedwith a PAPR/CM requirement. If the PAPR/CM requirement is not satisfied,in step S360, a random signal is generated for the reserved subcarriers,and a new Tx signal is generated by mapping the random signal to thereserved subcarrier. It is determined whether the new Tx signalsatisfies the PAPR/CM requirement. If the PAPR/CM requirement issatisfied, a lookup table is updated in step S370. The lookup tableincludes information regarding the modulation scheme, the sequence, thereserved subcarrier, and the random signal.

FIG. 14 shows an example of control signal transmission using a longspreading code.

Referring to FIG. 14, UE identification and/or cell identification usingthe spreading code in a time or frequency domain are determinedaccording to a length or characteristic of a code in use. A relativelyshort spreading code is not effective to remove inter-cell interferencesince it has generally a small cardinality of a code set. Therefore, thelong spreading code is necessary for further effective inter-cellidentification in a multi-cell environment. In a sequence (e.g., a ZCsequence), the PAPR/CM characteristic is maintained when all codes areused within a limited frequency resource. However, it is difficult todivide the sequence in the time domain and to map the sequence in thefrequency domain. Therefore, the long spreading code is segmented, andeach segment is subjected to DFT, subcarrier mapping, and IFFT. By doingso, an excellent PAPR/CM characteristic can be maintained. In addition,inter-cell identification can be easily achieved using the longspreading code.

Channel estimation can be more reliable when a reference signal isrobust to inter-cell interference in a CQI channel assigned with two RSOFDM symbols. UE capability is only 6 in the conventional CQI channelstructure. To obtain performance superior to that of the conventionalmethod in terms of intra-cell UE identification or inter-cellidentification, a ZC sequence having a double length may be segmented ina duration of two OFDM symbols

Although the descriptions above have focused on the CQI channel, thetechnical features of the present invention may also apply to varioustypes of control channels. For example, those skilled in the art will beable to easily apply the present invention to an ACK/NACK channel to bedescribed below.

FIG. 15 shows a structure of an ACK/NACK channel. The ACK/NACK channelis a control channel through which an ACK/NACK signal for HARQ istransmitted. The ACK/NACK signal is a transmission and/or receptionconfirm signal for downlink data.

Referring to FIG. 15, among 7 OFDM symbols included in one slot, a RS iscarried on 3 contiguous OFDM symbols in the middle portion of the slotand the ACK/NACK signal is carried on the remaining 4 OFDM symbols. TheRS is carried on the 3 contiguous OFDM symbols located in the middleportion of the slot. The location and the number of symbols used in theRS may vary depending on a control channel. Changes in the location andthe number the symbols may result in changes in those symbols used inthe ACK/NACK signal.

When the control signal is transmitted within a pre-assigned band,frequency-domain spreading and time-domain spreading are simultaneouslyused to increase the number of multiplexable UEs and the number ofcontrol channels. A frequency-domain spreading code is used to spreadthe ACK/NACK signal on a frequency domain. A ZC sequence may be used asthe frequency-domain spreading code. The frequency-domain spreadACK/NACK signal is subjected to IFFT processing and is then spread againin a time domain by using a time-domain spreading code. The ACK/NACKsignal is spread using 4 time-domain spreading codes wθ, w1, w2, and w3for 4 OFDM symbols. The RS is also spread using a spreading code havinga length of 3.

The present invention can be implemented with hardware, software, orcombination thereof. In hardware implementation, the present inventioncan be implemented with one of an application specific integratedcircuit (ASIC), a digital signal processor (DSP), a programmable logicdevice (PLD), a field programmable gate array (FPGA), a processor, acontroller, a microprocessor, other electronic units, and combinationthereof, which are designed to perform the aforementioned functions. Insoftware implementation, the present invention can be implemented with amodule for performing the aforementioned functions. Software is storablein a memory unit and executed by the processor. Various means widelyknown to those skilled in the art can be used as the memory unit or theprocessor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1-13. (canceled)
 14. A method of transmitting control signals in awireless communication system, the method comprising: generating aplurality of first modulation symbols by modulating a first controlsignal; generating a second modulation symbol by modulating a secondcontrol signal; generating a plurality of first spread signals bymultiplying each of the plurality of first modulation symbols to eachcyclic shift sequence, each cyclic shift sequence being generated bycyclically shifting a base sequence; generating a plurality of referencesignal (RS) sequences for a RS used for demodulating the first controlsignal; generating a second spread signal by multiplying the secondmodulation symbol to one of the plurality of RS sequences; andtransmitting the plurality of first spread signals and the second spreadsignal.
 15. The method of claim 14, further comprising: transmitting theremaining at least one RS sequence which is not multiplied to the secondmodulation symbol.
 16. The method of claim 14, wherein the number ofinformation bits for the first control signal is larger than the numberof information bits of the second control signal.
 17. The method ofclaim 16, wherein the first control signal is a channel qualityindicator (CQI) which represents a downlink channel condition and thesecond control signal is a positive acknowledgement (ACK) ornegative-acknowledgement (NACK) signal for a hybrid automatic repeatrequest (HARD).
 18. The method of claim 17, wherein the first controlsignal uses quadrature phase shift keying (QPSK) modulation and thesecond control signal uses QPSK or binary phase shift keying (BPSK)modulation.
 19. The method of claim 14, wherein the plurality of firstspread signals and the second spread signal are transmitted in differentorthogonal frequency division multiplexing (OFDM) symbols.
 20. Themethod of claim 14, wherein the plurality of RS sequences are generatedby cyclically shifting the base sequence.
 21. An apparatus oftransmitting control signals in a wireless communication system, thetransmitter comprising a processor configured for: generating aplurality of first modulation symbols by modulating a first controlsignal; generating a second modulation symbol by modulating a secondcontrol signal; generating a plurality of first spread signals bymultiplying each of the plurality of first modulation symbols to eachcyclic shift sequence, each cyclic shift sequence being generated bycyclically shifting a base sequence; generating a plurality of referencesignal (RS) sequences for a RS used for demodulating the first controlsignal; generating a second spread signal by multiplying the secondmodulation symbol to one of the plurality of RS sequences; andtransmitting the plurality of first spread signals and the second spreadsignal.
 22. The apparatus of claim 21, further comprising: transmittingthe remaining at least one RS sequence which is not multiplied to thesecond modulation symbol.
 23. The apparatus of claim 21, wherein thenumber of information bits for the first control signal is larger thanthe number of information bits of the second control signal.
 24. Theapparatus of claim 23, wherein the first control signal is a channelquality indicator (CQI) which represents a downlink channel conditionand the second control signal is a positive acknowledgement (ACK) ornegative-acknowledgement (NACK) signal for a hybrid automatic repeatrequest (HARD).
 25. The apparatus of claim 24, wherein the first controlsignal uses quadrature phase shift keying (QPSK) modulation and thesecond control signal uses QPSK or binary phase shift keying (BPSK)modulation.
 26. The apparatus of claim 21, wherein the plurality offirst spread signals and the second spread signal are transmitted indifferent orthogonal frequency division multiplexing (OFDM) symbols. 27.The apparatus of claim 21, wherein the plurality of RS sequences aregenerated by cyclically shifting the base sequence.